How best to find the "best" small solar panel of a particular size?
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because if you have a 5v solar panel you can charge it directly from the solar panel and then use an LDO to get the 3.3v or a buck-boost converter to always have 3.3v (in case you are using rfm69hw that requires a little higher voltage than rfm69h). Of course you could have a 5v->2.7v buck converter to charge the supercap and then use it to directly power your boards. My only biggest concern it to avoid overcharging the supercap, so using a 5.5V supercap you have a safe margin when using a 5V solar panel (could be a 6V panel and add 1 or 2 diodes to drop voltage a little )
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because if you have a 5v solar panel you can charge it directly from the solar panel and then use an LDO to get the 3.3v or a buck-boost converter to always have 3.3v (in case you are using rfm69hw that requires a little higher voltage than rfm69h). Of course you could have a 5v->2.7v buck converter to charge the supercap and then use it to directly power your boards. My only biggest concern it to avoid overcharging the supercap, so using a 5.5V supercap you have a safe margin when using a 5V solar panel (could be a 6V panel and add 1 or 2 diodes to drop voltage a little )
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I have seen theese too, it only gets more complex/expensive to make the circuit and source those special components
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@gohan Did you see this?
Click here@ceech is selling a few interesting solar harvesting boards.
Here's the thing: when you consider the "quantity 1" price of either the chip or Ceech's boards, you could boost your voltage more cheaply by just buying a bunch of cheap solar panels and stringing them together in series. For instance, today's Digikey price for the BQ25770 is $8.12: https://www.digikey.com/product-detail/en/texas-instruments/BQ25570RGRT/296-37014-1-ND/4430487
I'm not sure how they price these chips, but it seems like they price themselves out of much of the market to the point where they become almost like novelty items.
BTW, I like what I can see in Ceech's boards, and I think they're fairly priced considering the cost of the components. I'm just trying to be objective about this.
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Here's the thing: when you consider the "quantity 1" price of either the chip or Ceech's boards, you could boost your voltage more cheaply by just buying a bunch of cheap solar panels and stringing them together in series. For instance, today's Digikey price for the BQ25770 is $8.12: https://www.digikey.com/product-detail/en/texas-instruments/BQ25570RGRT/296-37014-1-ND/4430487
I'm not sure how they price these chips, but it seems like they price themselves out of much of the market to the point where they become almost like novelty items.
BTW, I like what I can see in Ceech's boards, and I think they're fairly priced considering the cost of the components. I'm just trying to be objective about this.
Likewise, I don't understand by the LTC3525D-3.3 (above) is about 4.5x the price of the MCP1642D-33 (also above), especially when the latter chip actually appears to be superior, at least on paper. Am I missing something?
[Edit: Answering my own question, it appears that the LTC3525D-3.3 has a much lower operational quiescent current. The MCP1642D-33's appears to be around 40x higher. So, that might definitely matter if you're planning to have the boost converter running all the time. ]
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I went ahead and made the switch on the node that I'm experimenting with: I'm now using a LTC3235D 3.3v passthrough boost converter. I switch it on when the node wakes up, do various measurements, then switch it off before transmitting the results if the supercap voltage is above threshold. Everything working smoothly.
I think I may next try putting a buck converter on the solar panel so I can get more juice out of it when its voltage is above 3.3v. I'll be trying this one:
https://www.openhardware.io/view/276/33v-Buck-Boost-DC-DC-Converter
because it's the only one I have on hand. If it seems to help, then I'll look for something more on-point (and less expensive!). -
I went ahead and made the switch on the node that I'm experimenting with: I'm now using a LTC3235D 3.3v passthrough boost converter. I switch it on when the node wakes up, do various measurements, then switch it off before transmitting the results if the supercap voltage is above threshold. Everything working smoothly.
I think I may next try putting a buck converter on the solar panel so I can get more juice out of it when its voltage is above 3.3v. I'll be trying this one:
https://www.openhardware.io/view/276/33v-Buck-Boost-DC-DC-Converter
because it's the only one I have on hand. If it seems to help, then I'll look for something more on-point (and less expensive!).UPDATE: I finally received these:
https://www.aliexpress.com/snapshot/0.html?orderId=83519564882273&productId=32644217312Even though they are smaller, under dim lighting, they showed ~4x the open circuit voltage as compared to: https://www.aliexpress.com/snapshot/0.html?orderId=83597824322273&productId=32253274314
That said, the panasonic solar panels (which are even smaller!) are about 4x their voltage (i.e. about 16x the 53mmx30mm ones) under the same dim lighting.
So, regarding the OP: yes, there are definitely quality differences that express themselves in dim lighting, and this is proof.
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UPDATE: I finally received these:
https://www.aliexpress.com/snapshot/0.html?orderId=83519564882273&productId=32644217312Even though they are smaller, under dim lighting, they showed ~4x the open circuit voltage as compared to: https://www.aliexpress.com/snapshot/0.html?orderId=83597824322273&productId=32253274314
That said, the panasonic solar panels (which are even smaller!) are about 4x their voltage (i.e. about 16x the 53mmx30mm ones) under the same dim lighting.
So, regarding the OP: yes, there are definitely quality differences that express themselves in dim lighting, and this is proof.
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@NeverDie Interesting find. Could you post the product links instead of the order links? Order links can't be visited without being logged in as the user who made the order.
@mfalkvidd said in How best to find the "best" small solar panel of a particular size?:
@NeverDie Interesting find. Could you post the product links instead of the order links? Order links can't be visited without being logged in as the user who made the order.
I can't post a link now to the good ones, because it looks as though they are no longer being sold. I had posted links before that worked. Believe me, no one is sorrier about this than me, because I want to order more of them.
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I just today started using a smart diode between the solar panel and the supercap. Voltage drop across it is a mere 0.03v, so I assume that means more power is faster transfered from the panel into the supercap.
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@NeverDie said in How best to find the "best" small solar panel of a particular size?:
smart diode
what's that?
@gohan said in How best to find the "best" small solar panel of a particular size?:
@NeverDie said in How best to find the "best" small solar panel of a particular size?:
smart diode
what's that?
It's this:
It better approximates an "ideal diode" than an actual diode does. So, it behaves like a diode, but the voltage drop is much less. I measured the reverse current leakage on it, and it's about 1ua, which is higher than some of the best diodes, but not bad.
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TI also makes a single integrated "smart diode": http://www.ti.com/product/SM74611
which I've also tried. However, it has a bigger voltage drop, so I don't like it as much. Also, TI's chip is more expensive. -
Back to the OP, here's a screen scrape of what had been the "good" small solar cells:
Here's a closeup photo of one:
It may not show so well in the photo, but what I think may be key is that it wasn't pieced together out of scrap solar cell fragments (like the "bad" one was) but rather appears to be a single piece. So, that may be a clue on what to look for in the future.
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Here's a schematic for the ideal diode that is pictured above:
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Not sure. There's probably a risk of UV damage to the epoxy, but that's just a guess.
At the moment, I'm focused on indoor use, which is a largely ignored category. I think outdoors is easier because, well, it's a lot brighter. For instance, novicit (above) reported "The solar panel is from China, Ebay, 110mmx70mm, 5V, 1.25W. The panel works great also. It reaches 4V in the smallest amount of light, and 5.45V (no load) in modest indirect light. In fact, it will fully recharge the supercapacitor in the morning before the sun even rises above the horizon, just from the sky brightening. (From the ~20% overnight discharged state.) It seems to charge a fully empty supercap in ~10 minutes under average light."
